The invention relates to a hip-joint prosthesis having a shaft, fastened on whose shaft head there is a ball head which in turn is inserted in a rotatable manner in the hemispherical recess of a socket insert, and the socket insert is coupled with a hip socket, wherein the shaft can be implanted in the femur, and the hip socket can be implanted in the pelvic bone.
A plurality of prosthetic systems for replacing a natural hip joint exists on the market. As a rule, these consist of a shaft 1 coupled with a ball head 2 and of a hip socket 4 coupled with a socket insert 3. The shaft 1 and the hip socket 4 are connected to the body as a result of growing into the femur (20) and pelvic bone (21) respectively and are carriers for the ball head 2 and the socket insert 3 respectively. The ball head 2 is rotatably mounted in the hemispherical recess of the socket insert 3—degree of freedom: 1 (see FIG. 1).
During the articulation of the ball head in the hemispherical recess of the socket insert, for various reasons and in particular when materials of high levels of hardness are used for the ball head and socket insert (e.g. metal alloys, ceramic materials), undesirable solid-body friction can occur between the sliding partners. Depending on the material pairing, surface structure and relative speed of the two friction partners, during the movement under the effect of solid-body friction a so-called stick-slip effect can occur. This means that the quasi-continuous movement of the ball head in the hemispherical recess, when looked at closely, is made up of many temporally very short movement cycles—in each case a short movement directly followed by sudden stoppage and in turn sudden movement.
This stick-slip effect is caused by constant alternation of static and sliding friction.
The vibrations emitted in consequence of the occurrence of the stick-slip effect act as excitation and lead to the vibration of the individual components of the artificial joint. If one or more of the characteristic frequencies of the components then lies/lie in the audible spectrum (approximately 16-20000 Hz), it/they can be perceived acoustically by the patient as the carrier of the artificial hip-joint prosthesis, for example in the form of so-called squeaking. This is undesirable for the patient, is possibly also perceived in his surroundings and, if applicable, leads to a considerable personal restriction.